linux-kernel - Re: [PATCH v2 02/18] clk: intel: Add clock driver for Intel MIPS SoCs

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Date:   Fri, 31 Aug 2018 10:10:50 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     "Zhu, Yi Xin" <yixin.zhu@...ux.intel.com>,
        Songjun Wu <songjun.wu@...ux.intel.com>,
        chuanhua.lei@...ux.intel.com, hua.ma@...ux.intel.com,
        qi-ming.wu@...el.com
Cc:     linux-mips@...ux-mips.org, linux-clk@...r.kernel.org,
        linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
        Michael Turquette <mturquette@...libre.com>,
        linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2 02/18] clk: intel: Add clock driver for Intel MIPS SoCs

Quoting Zhu, Yi Xin (2018-08-28 23:56:22)
> 
> On 8/28/2018 3:09 AM, Stephen Boyd wrote:
> > Quoting yixin zhu (2018-08-08 01:52:20)
> >> On 8/8/2018 1:50 PM, Stephen Boyd wrote:
> >>>> +/* clock flags definition */
> >>>> +#define CLOCK_FLAG_VAL_INIT    BIT(16)
> >>>> +#define GATE_CLK_HW            BIT(17)
> >>>> +#define GATE_CLK_SW            BIT(18)
> >>>> +#define GATE_CLK_VT            BIT(19)
> >>> What does VT mean? Virtual?
> >> Yes. VT means virtual here.
> >> Will change to GATE_CLK_VIRT.
> >>
> > Is it a hardware concept? Or virtualization with hypervisor?
> 
> Some peripheral drivers want to use same code cross platforms.
> 
> But not all platforms provide HW gate clock.  So in this case, clock 
> driver creates
> 
> a virtual gate clock to make it work if no HW gate clock in the SoC.

That's not how things are supposed to work. If a clk isn't there in the
hardware we don't make them up in software so that the consumer software
drivers can keep requesting clks on different platforms. On a different
platform, the driver needs to know that the clks aren't there with a
different compatible string.

> 
> 
> >
> >>>> +}
> >>>> +
> >>>> +CLK_OF_DECLARE(intel_grx500_cgu, "intel,grx500-cgu", grx500_clk_init);
> >>> Any reason a platform driver can't be used instead of CLK_OF_DECLARE()?
> >> It provides CPU clock which is used in early boot stage.
> >>
> > Ok. What is the CPU clock doing in early boot stage? Some sort of timer
> > frequency? If the driver can be split into two pieces, one to handle the
> > really early stuff that must be in place to get timers up and running
> > and the other to register the rest of the clks that aren't critical from
> > a regular platform driver it would be good. That's preferred model if
> > something is super critical.
> 
> Yes, CPU clock is providing CPU frequency in the early boot stage.
> 
> Will put the non-critical clocks in the platform driver.
> 
> 

Sure the CPU clock is handling frequency, but does that matter for early
boot to get going? If timers aren't involved here then it doesn't sound
like this needs CLK_OF_DECLARE.